GHSA-vmm3-xgcx-67hm

Suggest an improvement
Source
https://github.com/advisories/GHSA-vmm3-xgcx-67hm
Import Source
https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/08/GHSA-vmm3-xgcx-67hm/GHSA-vmm3-xgcx-67hm.json
JSON Data
https://api.osv.dev/v1/vulns/GHSA-vmm3-xgcx-67hm
Aliases
Published
2026-08-26T14:10:59Z
Modified
2026-08-26T14:26:28Z
Severity
  • 8.2 (High) CVSS_V4 - CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N CVSS Calculator
Summary
http4s has HTTP/2 Denial of Service with Ember Backend
Details

Summary

http4s 0.23.x and 1.0 servers running ember with http2 enabled are vulnerable to a denial of service attack using a HPACK bomb vulnerability recently disclosed as affecting other http2 servers.

Impact

Denial of Service:

  • Affects any http4s server running the ember backend with HTTP/2 enabled that is exposed to untrusted traffic.
  • Affects any http4s client running the ember backend with HTTP/2 enabled that can be directed to an untrusted server.

Details

The issue occurs in the Hpack wrapper. Ember concatenates the header and continuation frames before decoding all headers at once. When the code decodes the attack payload, the relatively small packets decode to a significantly larger amount of data which is returned as a single List which is then held in memory for further processing. With enough concurrent connections (~5 with a 2GB heap in testing) this leads to an OOM, eg:

java.lang.OutOfMemoryError: Java heap space
        at scala.collection.mutable.ListBuffer.scala$collection$mutable$ListBuffer$$freshFrom(ListBuffer.scala:129)
        at scala.collection.mutable.ListBuffer.addAll(ListBuffer.scala:147)
        at scala.collection.mutable.ListBuffer.addAll(ListBuffer.scala:40)
        at scala.collection.mutable.Growable.$plus$plus$eq(Growable.scala:69)
        at scala.collection.mutable.Growable.$plus$plus$eq$(Growable.scala:69)
        at scala.collection.mutable.AbstractBuffer.$plus$plus$eq(Buffer.scala:314)
        at org.http4s.Header$.org$http4s$Header$ToRaw$$anon$10$$_$$lessinit$greater$$anonfun$2(Header.scala:192)
        at org.http4s.Header$ToRaw$$anon$10$$Lambda/0x00001fe001239c50.apply(Unknown Source)
        at scala.collection.immutable.List.foreach(List.scala:334)
        at org.http4s.Header$ToRaw$$anon$10.<init>(Header.scala:192)
        at org.http4s.Header$ToRaw$.scalaCollectionSeqToRaw(Header.scala:195)
        at org.http4s.Headers$.apply(Headers.scala:220)
        at org.http4s.ember.core.h2.PseudoHeaders$.headersToRequestNoBody(PseudoHeaders.scala:95)
        at org.http4s.ember.core.h2.H2Stream.receiveHeaders$$anonfun$1$$anonfun$2$$anonfun$3$$anonfun$2(H2Stream.scala:248)
        at org.http4s.ember.core.h2.H2Stream$$Lambda/0x00001fe001511fd8.apply(Unknown Source)
        at cats.effect.IOFiber.runLoop(IOFiber.scala:429)
        at cats.effect.IOFiber.autoCedeR(IOFiber.scala:1460)
        at cats.effect.IOFiber.run(IOFiber.scala:129)
        at cats.effect.unsafe.WorkerThread.run(WorkerThread.scala:935)

Fix

The Hpack handling code has a configurable max header size parameter, but it does not include indexed headers in that accounting, allowing the attack to bypass the limit. There is not any configuration available in current http4s versions that can be enabled to prevent this attack short of disabling http2.

A fix has been tested locally that threads the maxHeaderSize setting from the server builder into the Hpack code. Using this, connections can be prematurely terminated once the decoded data size exceeds the maxHeaderSize that users can already configure.

Workarounds

If you can't upgrade immediately:

  • Disable HTTP/2 in Ember backends
Database specific 
{
    "cwe_ids":  [
        "CWE-409"
    ],
    "github_reviewed":  true,
    "github_reviewed_at":  "2026-08-26T14:10:59Z",
    "nvd_published_at":  null,
    "severity":  "HIGH"
}
References

Affected packages

Maven
org.http4s:http4s-ember-core_2.12

Package

Name
org.http4s:http4s-ember-core_2.12
View open source insights on deps.dev
Purl
pkg:maven/org.http4s/http4s-ember-core_2.12

Affected ranges

Type
ECOSYSTEM
Events
Introduced
0 Unknown introduced version / All previous versions are affected
Fixed
0.23.35

Affected versions

0.*
0.10.0-M10
0.21.0-M2
0.21.0-M3
0.21.0-M4
0.21.0-M5
0.21.0-M6
0.21.0-RC1
0.21.0-RC2
0.21.0-RC3
0.21.0-RC4
0.21.0-RC5
0.21.0
0.21.1
0.21.2
0.21.3
0.21.4
0.21.5
0.21.6
0.21.7
0.21.8
0.21.9
0.21.11
0.21.12
0.21.13
0.21.14
0.21.15
0.21.16
0.21.17
0.21.18
0.21.19
0.21.20
0.21.21
0.21.22
0.21.23
0.21.24
0.21.25
0.21.26
0.21.27
0.21.28
0.21.29
0.21.30
0.21.31
0.21.32
0.21.33
0.21.34
0.22.0-M1
0.22.0-M2
0.22.0-M3
0.22.0-M4
0.22.0-M5
0.22.0-M6
0.22.0-M7
0.22.0-M8
0.22.0-RC1
0.22.0
0.22-53-01128f5
0.22-96-55d3184
0.22-129-24d065b
0.22-143-49b5a8d
0.22.1
0.22.2
0.22.3
0.22.4
0.22.5
0.22.6
0.22.7
0.22.8
0.22.9
0.22.10
0.22.11
0.22.12
0.22.13
0.22.14
0.22.15
0.23.0-M1
0.23.0-RC1
0.23.0
0.23.1
0.23.2
0.23.3
0.23.4
0.23.5
0.23.6
0.23.7
0.23.8
0.23.9
0.23.10
0.23.11
0.23.12
0.23.13
0.23.14
0.23.15
0.23.16
0.23.17
0.23.18
0.23.19-RC1
0.23.19-RC2
0.23.19-RC3
0.23.19
0.23.20
0.23.21
0.23.22
0.23.23
0.23.24
0.23.25
0.23.26
0.23.27
0.23.28
0.23.29
0.23.30
0.23.31
0.23.32
0.23.33
0.23.34

Database specific

last_known_affected_version_range 
"<= 0.23.34"
source 
"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/08/GHSA-vmm3-xgcx-67hm/GHSA-vmm3-xgcx-67hm.json"
org.http4s:http4s-ember-core_2.13

Package

Name
org.http4s:http4s-ember-core_2.13
View open source insights on deps.dev
Purl
pkg:maven/org.http4s/http4s-ember-core_2.13

Affected ranges

Type
ECOSYSTEM
Events
Introduced
1.0.0-M1
Fixed
1.0.0-M47

Affected versions

1.*
1.0.0-M2
1.0.0-M3
1.0.0-M4
1.0.0-M5
1.0.0-M6
1.0.0-M7
1.0.0-M8
1.0.0-M9
1.0.0-M10
1.0.0-M11
1.0.0-M13
1.0.0-M14
1.0.0-M15
1.0.0-M16
1.0.0-M17
1.0.0-M18
1.0.0-M19
1.0.0-M20
1.0.0-M21
1.0.0-M22
1.0.0-M23
1.0.0-M24
1.0.0-M25
1.0.0-M27
1.0.0-M28
1.0.0-M29
1.0.0-M30
1.0.0-M31
1.0.0-M32
1.0.0-M33
1.0.0-M34
1.0.0-M35
1.0.0-M36
1.0.0-M37
1.0.0-M38
1.0.0-M39
1.0.0-M40
1.0.0-M41
1.0.0-M42
1.0.0-M43
1.0.0-M44
1.0.0-M45
1.0.0-M46

Database specific

last_known_affected_version_range 
"< 1.0.0-M46"
source 
"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/08/GHSA-vmm3-xgcx-67hm/GHSA-vmm3-xgcx-67hm.json"
org.http4s:http4s-ember-core_3

Package

Name
org.http4s:http4s-ember-core_3
View open source insights on deps.dev
Purl
pkg:maven/org.http4s/http4s-ember-core_3

Affected ranges

Type
ECOSYSTEM
Events
Introduced
0 Unknown introduced version / All previous versions are affected
Fixed
0.23.35

Affected versions

0.*
0.22.0-M8
0.22.0-RC1
0.22.0
0.22.1
0.22.2
0.22.3
0.22.4
0.22.5
0.22.6
0.22.7
0.22.8
0.22.9
0.22.10
0.22.11
0.22.12
0.22.13
0.22.14
0.22.15
0.23.0-M1
0.23.0-RC1
0.23.0
0.23.1
0.23.2
0.23.3
0.23.4
0.23.5
0.23.6
0.23.7
0.23.8
0.23.9
0.23.10
0.23.11
0.23.12
0.23.13
0.23.14
0.23.15
0.23.16
0.23.17
0.23.18
0.23.19-RC1
0.23.19-RC2
0.23.19-RC3
0.23.19
0.23.20
0.23.21
0.23.22
0.23.23
0.23.24
0.23.25
0.23.26
0.23.27
0.23.28
0.23.29
0.23.30
0.23.31
0.23.32
0.23.33
0.23.34

Database specific

last_known_affected_version_range 
"<= 0.23.34"
source 
"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/08/GHSA-vmm3-xgcx-67hm/GHSA-vmm3-xgcx-67hm.json"
org.http4s:http4s-ember-core_3

Package

Name
org.http4s:http4s-ember-core_3
View open source insights on deps.dev
Purl
pkg:maven/org.http4s/http4s-ember-core_3

Affected ranges

Type
ECOSYSTEM
Events
Introduced
1.0.0-M1
Fixed
1.0.0-M47

Affected versions

1.*
1.0.0-M22
1.0.0-M23
1.0.0-M24
1.0.0-M25
1.0.0-M27
1.0.0-M28
1.0.0-M29
1.0.0-M30
1.0.0-M31
1.0.0-M32
1.0.0-M33
1.0.0-M34
1.0.0-M35
1.0.0-M36
1.0.0-M37
1.0.0-M38
1.0.0-M39
1.0.0-M40
1.0.0-M41
1.0.0-M42
1.0.0-M43
1.0.0-M44
1.0.0-M45
1.0.0-M46

Database specific

last_known_affected_version_range 
"< 1.0.0-M46"
source 
"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/08/GHSA-vmm3-xgcx-67hm/GHSA-vmm3-xgcx-67hm.json"
org.http4s:http4s-ember-core_2.13

Package

Name
org.http4s:http4s-ember-core_2.13
View open source insights on deps.dev
Purl
pkg:maven/org.http4s/http4s-ember-core_2.13

Affected ranges

Type
ECOSYSTEM
Events
Introduced
0 Unknown introduced version / All previous versions are affected
Fixed
0.23.35

Affected versions

0.*
0.10.0-M10
0.21.0-M2
0.21.0-M3
0.21.0-M4
0.21.0-M5
0.21.0-M6
0.21.0-RC1
0.21.0-RC2
0.21.0-RC3
0.21.0-RC4
0.21.0-RC5
0.21.0
0.21.1
0.21.2
0.21.3
0.21.4
0.21.5
0.21.6
0.21.7
0.21.8
0.21.9
0.21.11
0.21.12
0.21.13
0.21.14
0.21.15
0.21.16
0.21.17
0.21.18
0.21.19
0.21.20
0.21.21
0.21.22
0.21.23
0.21.24
0.21.25
0.21.26
0.21.27
0.21.28
0.21.29
0.21.30
0.21.31
0.21.32
0.21.33
0.21.34
0.22.0-M1
0.22.0-M2
0.22.0-M3
0.22.0-M4
0.22.0-M5
0.22.0-M6
0.22.0-M7
0.22.0-M8
0.22.0-RC1
0.22.0
0.22-53-01128f5
0.22-96-55d3184
0.22-129-24d065b
0.22-143-49b5a8d
0.22.1
0.22.2
0.22.3
0.22.4
0.22.5
0.22.6
0.22.7
0.22.8
0.22.9
0.22.10
0.22.11
0.22.12
0.22.13
0.22.14
0.22.15
0.23.0-M1
0.23.0-RC1
0.23.0
0.23.1
0.23.2
0.23.3
0.23.4
0.23.5
0.23.6
0.23.7
0.23.8
0.23.9
0.23.10
0.23.11
0.23.12
0.23.13
0.23.14
0.23.15
0.23.16
0.23.17
0.23.18
0.23.19-RC1
0.23.19-RC2
0.23.19-RC3
0.23.19
0.23.20
0.23.21
0.23.22
0.23.23
0.23.24
0.23.25
0.23.26
0.23.27
0.23.28
0.23.29
0.23.30
0.23.31
0.23.32
0.23.33
0.23.34

Database specific

last_known_affected_version_range 
"<= 0.23.34"
source 
"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/08/GHSA-vmm3-xgcx-67hm/GHSA-vmm3-xgcx-67hm.json"